Polymer matrices for polymer solder hybrid materials

ABSTRACT

Embodiments of the present invention provide various polymeric matrices that may be used as a binder matrix for polymer solder hybrid thermal interface materials. In alternative embodiments the binder matrix material may be phophozene, perfluoro ether, polyether, or urethane. For one embodiment, the binder matrix is selected to provide improved adhesion to a variety of interfaces. For an alternative embodiment the binder matrix is selected to provide low contact resistance. In alternative embodiments, polymeric materials containing fusible and non-fusible particles may be used in application where heat removal is desired and is not restricted to thermal interface materials for microelectronic devices.

This is a Divisional Application of Ser. No. 13/090,855 filed Apr. 20,2011 which is Continuation Application of Ser. No. 11/825,400 filed Jul.6, 2007 now U.S. Pat. No. 7,967,942 issued Jun. 28, 2011 which is aDivisional Application of Ser. No. 10/358,526 filed Feb. 4, 2003 nowU.S. Pat. No. 7,252,877 issued Aug. 7, 2007.

FIELD

Embodiments of the invention relate generally to the field of thermalinterface material (TIM) for thermally coupling electronic components tothermally conductive members, and more specifically to alternative lowmodulus polymer matrices for polymer solder hybrid (PSH) materials.

BACKGROUND

Integrated circuits are typically manufactured on semiconductor wafersthat are then sawed (diced) into individual die. Typical microelectronicdevices experience internal heating during operation that may reach alevel such that the device no longer functions properly. To avoid suchoverheating, the die package may be thermally coupled to heatdissipation hardware (e.g., a heat sink and/or heat spreader). Attachinga heat sink to the die package requires that two solid surfaces bebrought into intimate contact. The solid surfaces are not smooth enoughto allow the desired contact. This is due to the microscopic hills andvalleys of the solid surfaces as well as to macroscopic non-planarity inthe form of a concave, convex, or twisted shape. As two such solidsurfaces are brought together, only a small percentage of the surfacesmake physical contact, with the remainder separated by a layer ofinterstitial air. Some heat is conducted from the die through the pointsof physical contact, but the majority must be transferred through theinterstitial air layer. Since air is a relatively poor thermalconductor, the interstitial air layer is replaced with a TIM to increasethe joint thermal conductivity and thus improve heat flow across theinterface. The TIM brings the die package into good thermal contact withthe heat dissipation hardware.

Various types of thermally conductive materials may be used as the TIMto eliminate air gaps from the interface including greases, reactivecompounds, elastomers, and pressure sensitive adhesive films. TIMs aredesigned to conform to surface irregularities, thereby eliminating airvoids, thus improving heat flow through the interface.

FIGS. 1A and 1B illustrate an exploded view of TIMs used in a typicalmicroelectronic device in accordance with the prior art. As shown inFIG. 1A, a die package 105 mounted on a printed circuit board (PCB) 110coupled by TIM 115 to a heat dissipation device, shown as heat sink 120.The heat sink 120 is typically aluminum and has fins as shown. The heatsink 120 may also include a fan, not shown (active heat sink). TIM 115brings the die package 105 into intimate contact with the heat sink 120.

As shown in FIG. 1B, the microelectronic device may include a heatspreader 116 to improve the efficiency of heat transfer from the diepackage 105 to the heat sink 120. For such a device, a first TIM 115 maybe used to improve thermal contact between the die package 105 and theheat spreader 116. A second TIM 117 may be used to improve contactbetween the heat spreader 116 and the heat sink 120.

FIG. 2 illustrates an arrangement of a non-fusible particle fillermaterial within the polymer matrix of a TIM in accordance with the priorart. The polymer matrix may be a material that can be applied as a pastesuch as a dispensable syringe or by screen-printing. The polymer matrixmay also act as an adhesive to bond the two mating parts together. Thenon-fusible particles, such as most metals, benefit from a high thermalconductivity, however a thermal flow path through the TIM is limited bythe point-to-point contact of the particles as shown by the arrows.Non-fusible particles refer to particles that will not melt and flowduring packaging assembly process, reliability testing, and productoperation and so remain as point contacts with each other. This providesthermal conductivity through the TIM that is limited to point-to-pointpercolation, resulting in a thermal bottleneck through the non-fusibleparticles.

The phenomenon of percolation describes the effects of interconnectionspresent in a random system, here the number of filler particles that arerandomly in point contact with each other to allow thermal conduction.Normally, to improve conduction limited by percolation, the amount offiller could be increased until a threshold amount is reached and heatconduction, due to the filler, transitions to a sufficiently high value.The degree of filler required to reach this transition level may be toohigh and can overpower the properties desired from the polymer bindersuch as low contact resistance. Another problem is that for some metalparticles in contact with some polymer binders, the bare particle fillercan poison the polymer cure such as by hindering or blocking the curingagent.

To address these concerns, a PSH TIM has been developed that includesfusible particles as well as filler particles in a silicone polymermatrix material. The fusible particles melt during the assembly processand can therefore wet the filler particles or self-coalesce. Thereby theaverage particle size grows creating long continuous heat transferpathways that alleviate the thermal bottleneck of percolation. Thefusible particles may be materials such as solder-like materials thatmelt below approximately 300° C. The filler particles may be non-fusiblematerials with melting points well above 300° C., such as aluminum at660° C., silver at 961° C., copper at 1084° C., gold at 1064° C., etc.

Silicone exhibits certain characteristics (e.g., low glass transitiontemperature and low moisture absorbency) that make it suitable as abinder matrix for PSH TIMs. For purposes of this disclosure, low glasstransition temperature is approximately 25° C. and low moistureabsorbency is approximately 1% or less by weight. Other materials mayexhibit such characteristics and therefore may likewise be suitable as abinder matrix for PSH TIMs. Such materials may provide better adhesionand lower contact resistance than silicones and similar materials.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. In the drawings:

FIGS. 1A and 1B illustrate an exploded view of TIMs used in a typicalmicroelectronic device in accordance with the prior art;

FIG. 2 illustrates an arrangement of a non-fusible particle fillermaterial within the polymer matrix of a TIM in accordance with the priorart;

FIG. 3 illustrates a TIM that is inserted between and used for thermalcoupling of an electronic component to a thermally conductive member inaccordance with one embodiment of the invention;

FIG. 4 illustrates the agglomeration of the fusible solder particles inaccordance with one embodiment of the invention;

FIG. 5 illustrates the structure of phosphozene indicating suitabilityas a binder matrix for PSH TIMs;

FIGS. 6A and 6B illustrate the structure of perfluoro ether resinindicating suitability as a binder matrix for PSH TIMs;

FIGS. 7A and 7B illustrate alternative, exemplary backbone structures ofpolyether-based resin indicating suitability as a binder matrix for PSHTIMs; and

FIG. 8 illustrates an assembly employing an alternative binder matrixmaterial in a PSH TIM in accordance with one embodiment of theinvention.

DETAILED DESCRIPTION Overview

Embodiments of the present invention provide various polymeric matricesthat may be used as a binder matrix for PSH TIMs. For one embodiment,the binder matrix is selected to provide improved adhesion to a varietyof interfaces. For an alternative embodiment the binder matrix isselected to provide low contact resistance. In alternative embodiments,polymeric materials containing fusible and non-fusible particles may beused in application where heat removal is desired and is not restrictedto TIM for microelectronic devices.

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearance of the phrases “in one embodiment” or “in an embodiment” invarious places throughout the specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 3 illustrates a TIM that is inserted between and used for thermalcoupling of an electronic component 312 to a thermally conductive member314 in accordance with one embodiment of the invention. The thermalinterface material 310 includes a polymer matrix material 316, fusiblesolder particles 318 in the matrix material 316, and filler particles320 in the matrix material 316. The solder particles 318 have a meltingtemperature below a selected temperature and the filler particles 320have a melting temperature above the selected temperature. The solderparticles 318 will thus melt when the temperature increases to above theselected temperature but the filler particles 320 will not melt.

The matrix material 316 may comprise between 1% and 10% of the thermalinterface material 310 by weight and preferably comprises approximately8% by weight.

The solder particles 318 may comprise between 1% and 99% of the thermalinterface material 310 by weight, preferably at least 5% by weight, andmore preferably between 25% and 90% by weight.

The solder particles 318 preferably have a melting temperature ofbetween 60° C. and 300° C. The solder particles 318 may be made of puresolder compositions such as indium (In) with a melting temperature of157° C. or a solder alloy, indium tin (InSn) with a eutectic meltingtemperature of 118° C., indium silver (InAg) with a eutectic meltingtemperature of 139° C., tin silver (SnAg) or tin silver copper (SnAgCu)with a eutectic melting temperatures of 217° C., tin bismuth (SnBi) witha eutectic melting temperature of 203° C., indium tin bismuth (InSnBi)with a melting temperature of between 60° C. and 140° C., indiumtitanium (InTi), indium zirconium (InZr), indium titanium ceriumselenium (InTiCeSe), indium silver titanium cerium selenium(InAgTiSeCe), with melting temperatures between 145° C. to 165° C., etc.

The solder particles 318 may have diameters of between 0.2 and 100. Thesolder particles 318 may be a mixture of fine and coarse particles. Inalternative embodiments, the solder particles may be any of variousshapes including solder shavings.

The filler particles 320 may comprise between 0% or 95% of the thermalinterface material 310 by weight, more preferably at least 10% byweight.

The solder particles 318 and the filler particles 320 togetherpreferably comprise between 50% and 99% of the thermal interfacematerial 310 by weight, and preferably comprise approximately 92% byweight.

The filler particles 320 (either fusible, non fusible or ceramicparticles) preferably have a melting temperature above 350° C. and morepreferably between 800° C. and 1200° C. The filler particles 320preferably have a melting temperature that is at least 100° C., morepreferably at least 200° C. above a melting temperature of the solderparticles 318. The filler particles 320 may be nickel (Ni), copper (Cu)with a melting temperature of 1084° C., silver (Ag) with a meltingtemperature of 961° C., silver copper (Ag/Cu), tin (Sn), and graphite,and preferably are aluminum (Al) with a melting temperature of 660° C.Example of non-fusible fillers would be boron nitride, aluminum nitride,silicon carbide, aluminum oxide, graphite, carbon fiber, carbonnanotubes or diamond particles.

The matrix material 316 may be a phosphozene, a polyether, a urethane ora perfluoroether.

The whole assembly, including the electronic component 312, thethermally conductive member 314 and the thermal interface material 310is inserted into a furnace which heats the assembly from roomtemperature to a temperature above which the solder particles 318 melt.For example, the composition is heated from room temperature of about30° C. to approximately 170° C., which is above the melting temperatureof the solder particles so that the solder particles 318 melt. Thesolder particles 318 fuse and agglomerate together. FIG. 4 illustratesthe agglomeration of the fusible solder particles in accordance with oneembodiment of the invention. The temperature to which the assembly isheated is, however, maintained below a temperature at which the fillerparticles 320 melt. For example, the composition is maintained at 170°C. for approximately two minutes, (i.e. until sufficient agglomerationhas occurred). The assembly is then cooled to a temperature below themelting temperature of the solder particles 318 so that they solidify.For example, the composition is then cooled to a temperature ofapproximately 125° C. which is below the solder material's melting pointand the solder particles solidify. The temperature is further lowered toa selected temperature above room temperature at which the matrixmaterial 316 cures. Cross-linking may occur between polymer chains ofthe matrix material 316 while it is being cured to enhance theviscoelastic properties of the matrix material 316. The curing time andtemperature are related and vary with the type of polymer binder matrix.

The temperature is then further lowered to room temperature. In theresulting structure, the solder particles 318 are agglomerated togetherand have large surfaces contacting both the electronic component 312 andthe thermally conductive member 314 so as to provide an unbroken paththrough which heat can conduct from the electronic component 312 throughthe now consolidated solder particles 318 to the thermally conductivemember 314. The matrix material 316 has the ability to absorb stresseson the material. However, without the filler particles 320, the thermalinterface material 310 may tend to flow out from between the electroniccomponent 312 and the thermally conductive member 314 during thermalcycling and/or when exposed to high humidity. The filler particles 320provide the necessary strength to prevent such flow. The fillerparticles 320 thus keep the thermal interface material 310 intact duringadverse stress and thermal conditions.

Alternative Polymer Matrices

The polymer matrix material may be selected for various characteristics.Typically these characteristics include low, low moisture absorbency,adhesion to a variety of interfaces, low contact resistance, orcombinations thereof Several polymer matrices have been identified thatexhibit one or more of the above-noted characteristics and areconsidered suitable as a binder matrix for PSH TIMs.

Phosphozene

Phosphozenes are elastomeric materials (polymers with elastic propertiessimilar to natural rubber). Phosphozenes are a low glass transitiontemperature, low modulus (approximately 1 Gpa or less), materials thatare thermally stable. FIG. 5 illustrates the structure of phosphozene500 indicating suitability as a binder matrix for PSH TIMs. As shown inFIG. 5, phosphozene structure 500 includes a phosphorous/nitrogenbackbone 505 with attached substituents R 510 (where R may be, forexample, OCH₃, Oet, Oalkyl, Operfluoroalkyl, etc.). Phosphozenes aresimilar to siloxanes, but are more polar. This characteristic providesgood wetting of the interfaces and therefore lower contact resistance.The phosphorous and nitrogen, present in the backbone provide improvedadhesion, as well.

Moreover, the moisture absorbency of the polymer can be adjusted bymanipulation of the R-groups so it is possible to obtain a low moistureabsorbency binder matrix as desired.

Upon application of a catalyst and heat, phosphozene polymerizes(structure 515) like silicone, although the structure is different.

Perfluoro Ether Resin

Perfluoro ether resin is another resin type that likewise exhibitsdesirable characteristics for application as a PSH TIM. FIGS. 6A and 6Billustrate the structure of perfluoro ether resin indicating suitabilityas a binder matrix for PSH TIMs. FIG. 6A illustrates an example of aperfluoro ether resin, namely polyperfluoro propylene oxide resin,structure 600A. Perfluoro ether resins are elastomeric with low surfaceenergy, as well as thermally stable. Additionally, perfluoro etherresins can be designed to have low moisture absorbency and becross-linkable as shown in FIG. 6B. Structure 600B shows an example ofcross-linkable perfluoro ether resin. Structure 605B is the cross-linkedstructure created by subjecting the perfluoro ether resin to a catalystand heat.

Polyether-Based Resin

Polyether-based resin is another resin type that likewise exhibitsdesirable characteristics for application as a PSH TIM. FIGS. 7A and 7Billustrate alternative, exemplary backbone structures of polyether-basedresin, 700A and 700B, respectively, indicating suitability as a bindermatrix for PSH TIMs.

Polyethers are relatively inexpensive, amorphous, and can be designed tohave low moisture absorbency. Polyethers, too, can be madecross-linkable or non-cross-linkable and are easily controllable withrespect to polymerization.

Urethanes

Urethanes with a NHCOO linkage are elastomeric materials that likewiseexhibits desirable characteristics for application as a PSH TIM.Urethanes, in addition to low glass transition temperature and lowmoisture absorbency are especially robust. Moreover, urethanes areflexible in application in that texture and hardness may be easilyadjusted by varying the particular monomers used.

General Matters

Binder matrix polymers, as discussed above, provide one or moredesirable characteristics for use as PSH TIMs. These characteristicsinclude low glass transition temperature to provide more uniform heatdissipation, low moisture absorbency to avoid interface degradation, aswell as better adhesion and lower contact resistance. Additionally, thealternative binder matrix materials may provide a less expensive morerobust material that performs better than typical prior art bindermatrix materials.

FIG. 8 illustrates an assembly employing an alternative binder matrixmaterial in a PSH TIM in accordance with one embodiment of theinvention. Assembly 800 includes an electronic component 812, thethermally conductive member 814, and the PSH TIM 810. The electroniccomponent 812 is a semiconductor die (hereafter referred to as “die812”) having an integrated circuit formed in and on a lower surfacethereof. Solder bump contacts 832 are formed on the integrated circuit.The assembly 800 further includes a package substrate 834 having contactpads (not shown) on an upper surface thereof. Each contact 832 islocated on a respective contact pad. The combination of the packagesubstrate 834 and the die 812 is then inserted into a furnace so thatthe contacts 832 melt, and is then cooled so that the contacts 832secure the die 812 to the package substrate 834.

The thermally conductive member 814 is made of metal or ceramic andforms part of a metal cap having sides 836 extending downwardly fromedges of the thermally conductive member 814, past the die 812, to thesubstrate 834. The thermal interface material 810 is in the form shownin FIG. 3 when the cap is located over the die 812. Only then is theassembly 800 placed in a furnace so as to transform the thermalinterface material 810 into the form shown in FIG. 4.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. For example, though described above in reference to heat removalfrom a microelectronic device, various embodiments of the invention maybe implemented in a wide variety of heat removal applications (e.g.,automotive and optical chips). The description is thus to be regarded asillustrative instead of limiting.

What is claimed is:
 1. A method comprising: using a thermal interfacematerial to thermally couple a die to a thermally conductive member, thethermal interface material comprising a polymeric matrix material,fusible particles within the matrix material, the fusible particleshaving a melting point below a specified temperature, and non-fusibleparticles in the matrix material, the non-fusible particles having amelting point above the specified temperature; subjecting the thermalinterface material to a temperature above the specified temperature suchthat the fusible particles melt and agglomerate forming heat conductionpaths having surfaces contacting both the die and the thermal conductivemember so as to provide an unbroken path through which heat can conductfrom the die through the melted and agglomerated fusible particles tothe thermal conductive member.
 2. The method of claim 1 wherein thematrix material is cross-linkable.
 3. The method of claim 1 furthercomprising: curing the matrix material such that cross-linking occurs.4. The apparatus of claim 1 wherein the polymeric matrix material formspockets in the fused and agglomerated fusible particles.
 5. Theapparatus of claim 1 wherein the polymeric matrix material pocketscontains the non-fusible particles.